As stated by Barrett in his U.S. Pat. No. 4,718,289 which issued Jan. 12, 1988 for a Drill Cuttings Sample Collector, exploratory wells are drilled to evaluate penetrated rock formations for producible hydrocarbons. In drilling such wells (and in drilling production wells), circulation fluid is pumped down a string of drill pipe and through a drill bit at the lower end thereof. Such fluid thereafter circulates upwardly in the annulus between the drill pipe and the wellbore and thus flushes drill cuttings, such including drilled rock grains and fragments, from the wellbore.
In the usual operation, the drill cuttings are separated from the circulation fluid at the surface of the wellbore. The circulation fluid is thereafter recirculated through the drill string. Typically, such separation is done by a vibratory screening device; what is referred to in the industry as a shale shaker. The shale shaker comprises one or more vibrating screens. Fluid containing the drill cuttings is poured onto the screen(s) which permit the fluid to pass therethrough for recirculation. The screen(s) are inclined and vibrate the drill cuttings off one end, that is the lower end thereof. The drill cuttings are disposed of by burial on land or maybe put in a barge when drilling offshore.
Samples of drill cuttings which are separated from circulation fluid as described above are examined by geologists to evaluate the penetrated rock formations. Such examination is particularly important in connection with exploratory wells which are drilled for the purpose of determining the nature of the formation. It is important to collect samples from all penetrated depths so that the lithology of the formations from the surface of the well to the bottom of the bore may be accurately determined. The importance of such samples are increased when, after drilling is complete, a wireline log cannot be acquired or is of poor quality because of a damaged bore. In such cases, the cutting samples and related hydrocarbon records are the only results to show for the expense of drilling the well. It is desirable to screen out larger cuttings, which are typically referred to as cavings, and which are less likely to have been drilled during the interval of interest. Smaller sized cuttings are more likely to have been transported up the annulus at the same rate as the drilling fluid, the lag rate of which can be determined by the geologist.
As reported by Barret, in the past drill cuttings samples were collected by placing a container in the flow of drill cuttings which fell from lower end of the shale shaker. The container was periodically emptied and collected samples examined. Such prior art sample collectors suffered from several disadvantages. First, the container in which the samples accumulated would often fill to the top and overflow. Drill cuttings falling from the shale shaker when the container was full were not sampled and the record for such drill cuttings lost. This was especially true in the case of a drilling operation which was drilling at a fairly rapid rate and thus generating a high volume of drill cuttings per unit time.
Loss of drill cutting samples may also occur when fluids are accidentally introduced into the container collecting the samples. Such may occur as a result of a blinded shaker screen; that is, the screen becomes clogged and permits circulation fluid to flow with the drill cuttings into the container. Samples may also be lost as a result of water from a hose, which is used to clean the shale shaker, entering the container. In addition, heavy rain and, in offshore drilling operations, waves may introduce water into the container, thus greatly reducing or destroying the information obtainable from the samples.
The present invention addresses the problem in the prior art where often drill cuttings are allowed to accumulate in a stationary collector underneath the downstream end of the shale shaker, the problem being that sampling from such a collector quite often means an assistant merely scooping cuttings from the top of the collector or digging into the layered contents of the collector, whether or not mixed so that the actual sample taken by the assistant is not necessarily representative of the particular drilling interval which it is desired to sample.
As disclosed by Barrett in his patent referred to above, it is known to place a container under the outflow end of a so-called shale shaker, itself conventionally used in the art for removing drilling fluid from drilling cuttings. The container of Barrett is placed so as to capture drilling cuttings falling from the end of the shale shaker which as known in the art is a vibratory inclined screen conveyor which transports drill cuttings down along the inclined surface by means of vibrating the otherwise stationary surface of the screen whilst allowing circulation fluid to pass through the vibratory surface for collection and reuse. What Barrett fails to disclose, and which is one object of the present invention to provide, is an improved method and apparatus for screening, filtering or culling a selectively smaller volume of the drill cuttings exiting the shale shaker so that the sample still represents a uniform sampling of the desired drilling cross-section exhibited by the drilling cuttings exiting the shale shaker.
Other prior art of which applicant is aware include:
Canadian Patent No. 1,280,740 which issued to Hoeft on Feb. 26, 1991 for a Chip Sampler. Canadian Patent No. 1,079,229 which issued to Huff on Jun. 10, 1980 for a Mud Metering Testing and Cleansing System. U.S. Pat. No. 5,571,962 which issued to Georgi on Nov. 5, 1996 for a Method and Apparatus for Analyzing Drill Cuttings. U.S. Pat. No. 5,372,037 which issued to Butt on Dec. 13, 1994 for Soil Sampling Apparatus. U.S. Pat. No. 3,563,255 which issued to Morris on Feb. 16, 1971 for an Apparatus for Collecting and Washing Well Cuttings. U.S. Pat. No. 3,135,685 which issued to Tanner on Jun. 2, 1964 for a Device for Collecting Cutting Samples From Well Drilling Operations. U.S. Pat. No. 2,336,539 which issued to Gilbert on Dec. 14, 1943 for a Sampling Device.